Can't say I ever tuft-tested the decklid. I will do my best to capture that vital information in the coming days, as it will be key for the design of the extension. If and that is a BIG
if, flow does not re-attach at the decklid, would resorting to homemade vortex generators help the air flow, especially in the roof-to-glass transition ? I know that they are very hard to tweak but could they improve the flow when combined with a decklid extension. The rear glass is steep indeed.
Anyhow, I will come up with a tuft-testing video and we will know for sure.